1. Field of the Invention
This invention relates to the fabrication of magnetic read/write heads that employ TAMR (thermally assisted magnetic recording) to enable writing on magnetic media having high coercivity and high magnetic anisotropy. More particularly, it relates to the use of a plasmon antenna (PA) to transfer the required thermal energy from the read/write head to the media.
2. Description of the Related Art
Magnetic recording at area data densities of between 1 and 10 Tera-bits per in2 involves the development of new magnetic recording media, new magnetic recording heads and, most importantly, a new magnetic recording scheme that can delay the onset of the so-called “superparamagnetic” effect. This latter effect is the thermal instability of the extremely small regions on which information must be recorded, in order to achieve the required data densities. A way of circumventing this thermal instability is to use magnetic recording media with high magnetic anisotropy and high coercivity that can still be written upon by the increasingly small write heads required for producing the high data density. This way of addressing the problem produces two conflicting requirements:
1. The need for a stronger writing field that is necessitated by the highly anisotropic and coercive magnetic media.
2. The need for a smaller write head of sufficient definition to produce the high areal write densities, which write heads, disadvantageously, produce a smaller field gradient and broader field profile.
Satisfying these requirements simultaneously may be a limiting factor in the further development of the present magnetic recording scheme used in state of the art hard-disk-drives (HDD). If that is the case, further increases in recording area density may not be achievable within those schemes. One way of addressing these conflicting requirements is by the use of assisted recording methodologies, notably thermally assisted magnetic recording, or TAMR.
The prior art forms of assisted recording methodologies being applied to the elimination of the above problem share a common feature: transferring energy into the magnetic recording system through the use of physical methods that are not directly related to the magnetic field produced by the write head. If an assisted recording scheme can produce a medium-property profile to enable low-field writing localized at the write field area, then even a weak write field can produce high data density recording because of the multiplicative effect of the spatial gradients of both the medium property profile and the write field. These prior art assisted recording schemes either involve deep sub-micron localized heating by an optical beam or ultra-high frequency AC magnetic field generation.
The heating effect of TAMR works by raising the temperature of a small region of the magnetic medium to essentially its Curie temperature (TC), at which temperature both its coercivity and anisotropy are significantly reduced and magnetic writing becomes easier to produce within that region.
In the following, we will address our attention to a particular implementation of TAMR, namely the transfer of electromagnetic energy to a small, sub-micron sized region of a magnetic medium through interaction of the magnetic medium with the near field of an edge plasmon excited by an optical frequency laser. The transferred electromagnetic energy then causes the temperature of the medium to increase locally.
The edge plasmon is excited in a small conducting plasmon antenna (PA) approximately 200 nm in width that is incorporated within the read/write head structure. The source of optical excitement can be a laser diode, also contained within the read/write head structure, or a laser source that is external to the read/write head structure, either of which directs its beam of optical radiation at the antenna through a means of intermediate transfer such as an optical waveguide (WG). As a result of the WG, the optical mode of the incident radiation couples to a plasmon mode in the PA, whereby the optical energy is converted into plasmon energy, This plasmon energy is then focused by the PA onto the medium, at which point the heating occurs. When the heated spot on the medium is correctly aligned with the magnetic field produced by the write head pole, TAMR is achieved. The following prior arts describe such TAMR implementations in various forms.
K. Shimazawa et al. (US Publ. Pat. App. US2010/0103553) describes TAMR structures that utilize edge plasmon mode coupling. Shimazawa et al. shows a near-field light generator composed of an electroconductive material such as Au. No magnetic materials are disclosed.
Rochelle, (U.S. Pat. No. 6,538,617) describes an antenna for sensing magnetic fields that employs a ferrite magnetic core.
Takagishi et al. (US Publ. Pat. App. 2010/0027161) discloses an antenna having two magnetic layers with a noble metal layer between them.
Komura et al. (US Publ. Pat. Appl. 2009/0201600) teaches improving plasma generation efficiency by means of a V-shaped groove and a projection facing the deepest part of the groove in a structure formed of non-magnetic materials.
Y. Zhou et al. (US Appl. # U.S. Ser. No. 12/456,290 (2009) discloses a plasmon antenna with a magnetic core for thermally assisted magnetic recording.
None of these prior arts address the issues to be dealt with by the present invention, as will now be described in greater detail.
Referring first to FIG. 1, there is shown a schematic illustration of an exemplary prior art TAMR structure in an ABS (shown as a dashed line) view and in a side cross-sectional view. The dimensional directions in the ABS view are indicated as x-y coordinates (in the ABS plane), with the x coordinate being a cross-track coordinate in the plane of the medium and the y coordinate being a down-track direction. In the vertical (y-direction) cross-sectional view, the x coordinate would emerge from the plane of the drawing and the z coordinate is in the direction towards the ABS of the head (the “distal” direction).
The conventional magnetic write head includes a main magnetic pole (MP) (1), which is shown with a rectangular ABS shape, a writer coil (5) (three winding cross-sections drawn) for inducing a magnetic field within the pole structure and a return pole (3). Generally, magnetic flux emerges from the main magnetic pole, passes through the magnetic media and returns through the return pole.
The optical waveguide (WG) (4) guides optical frequency electromagnetic radiation (6) towards the air bearing surface (ABS) of the write head. The ABS end of the write head will also be called its distal end and the ends of all components that are closest to the ABS will be called their distal ends. The plasmon antenna (PA) (2), which has a triangular shape in the ABS plane, extends distally to the ABS and is adjacent to the MP (1). The distal end of the waveguide (4) is not at the ABS, but terminates at a depth, d, away from the ABS. An optical frequency mode (6) of the electromagnetic radiation couples to the edge plasmon mode (7) of the PA (2) and energy from the edge plasmon mode is then transmitted to the media surface where it heats the surface locally at the ABS edge of the PA triangle.
An advantage of the design illustrated in this figure is that the WG (4) terminates before reaching the ABS of the write head so that leakage of visible radiation to the ABS is reduced. Meanwhile, the energy from the edge plasmon mode (7), upon reaching the ABS, can achieve a spatially confined region that is desirable for achieving a high thermal gradient in the magnetic medium. With the long PA body (2) and large volume of metal composing the PA, heating damage of the PA is also greatly reduced.
In the prior art cited above, the materials used to form the PA are metals like Ag and Au that are known to be excellent in generating optically driven plasmon modes. However, in the prior art a problem still exists in aligning the optical heating profile within the region of energy transfer at the medium surface, with the magnetic field profile generated by the write head.
Referring to FIG. 2, there are shown schematically a typical prior art magnetic field profile (8) and below it, a heating profile (9), such as would be produced by the TAMR writer of FIG. 1 at the position of the heating spot (the peak of the profile) on the magnetic medium. The horizontal coordinate axis in both graphs is the y-coordinate of FIG. 1. The vertical axis is the magnetic field, Hz, in the magnetic field profile and the heat intensity, Pheat, in the heating profile. Both profiles are localized within a small region of the magnetic medium. For reference purposes, the ABS shape of the PA (2) and the ABS shape of the MP (1) (also shown in FIG. 1) are drawn below the axes, so the location of the field and heat transfer can be ascertained.
As can be seen in FIG. 2, the heating spot is at the far leading edge of the magnetic field profile produced by the MP. Although this location will allow sufficient writing resolution with enough heating, it is not the optimal positioning of the two curves relative to each other. To obtain the full benefit of TAMR, the slope of the heating profile (9) should be aligned with the encircled regions of maximum slope (10) or (11), of the magnetic field profile. In this case, a multiplicative factor of the two maximum gradients is obtained.
Due to structural limitations, caused, for example, by the thickness and arrangement of the WG and by choice of the PA design, difficulties in alignments during fabrication, etc., optimal alignment of the heating and field profiles cannot be obtained.
Referring to FIG. 3A, there is shown a schematic illustration of a front view (looking up at the ABS) of a portion of a simplified version of an alternative form of TAMR prior art as disclosed by Zhou et al. (cited above). The figure shows the ABS of the plasmon antenna (22) and the distal face (recessed from the ABS) of the adjacent optical waveguide (23). The plasmon antenna (22) has a core (24) formed of magnetic material, partially surrounded by a layer (27) (shown shaded for clarity) of a non-magnetic highly conductive metal (such as Au or Ag). The antenna is formed in the shape of an elongated prism (more clearly illustrated in the following figure), here shown as a prism with an (exemplary) triangular, or approximately triangular cross-section. We shall hereinafter call such an antenna, with its core of magnetic material, an MCA (magnetic core antenna).
Referring to FIG. 3B, there is shown a schematic perspective view of the same system as in FIG. 3A. The position of the antenna (22) with its vertex just above a face of the waveguide (23) promotes coupling of the edge plasmon (7), which is substantially confined to the vertex region of the conductive coating (27), to the electromagnetic optical mode (6) within the waveguide. The magnetic core (24) of the plasmon antenna serves to channel the magnetic flux of the main writer pole (not shown in this figure) so that it will align optimally with the thermal energy profile produced by the plasmon field within the magnetic medium.
Referring to FIG. 4, there is shown a schematic illustration of a side cross-sectional view of a particular arrangement of the type of MCA TAMR head structure already shown in FIG. 3B. In this illustration the main pole (21) of a magnetic writer has affixed (or adjacently mounted) to it the magnetic core plasmon antenna (22) (MCA) of the previous invention of FIG. 3B. The MCA (22) and main pole (MP) (21) share a common ABS (shown as a dashed line). A waveguide (WG) (23) is mounted adjacent to the antenna, MCA (22), and recessed vertically relative to the ABS. A schematic illustration of the ABS face of the MCA is shown encircled with a dashed line, to indicate the magnetic core (24), such as a core of FeCo or NiFe, partially overcoated with a layer (27) of Au (shown shaded for emphasis). In this configuration the flat face of the MCA, which is opposite the vertex and not covered by the overcoat (27), is parallel to the trailing edge of the MP, while the vertex of the MCA, which supports the edge plasmon mode, faces away from the trailing edge of the MP and is immediately adjacent to the WG (23). The WG is downtrack of the MCA and its distal end is vertically above the ABS. Dashed arrows from WG (23) to MCA (22) indicate the coupling of radiation from WG to the MCA. Arrows indicate the magnetic field emanating from both the pole, MP, (21) and antenna (22) and plasmon energy being emitted from the antenna as well. Of course the magnetic field from the antenna is emitted by its core (24), and the plasmon energy is emitted from its overcoat (27).
During recording, the magnetic field produced by the MP (21) magnetizes the core of the MCA (24) and can even saturate the core if the spacing is small, literally zero spacing being quite appropriate. Thus, the magnetic core of the antenna can be considered a part of the MP structure rather than the MCA structure, in that its role is to direct magnetic flux to the spot on the medium being heated rather than contribute to the heat generating properties of the edge plasmon mode.
Referring to FIG. 5, there is shown a graphical simulation of the magnetic field distribution of the pole (21) of FIG. 4, with two curve segments showing the distribution in both the presence (20) (solid line) and absence (25) (dashed line) of the MCA. In the simulation, the absent MCA actually corresponds to a plasmon antenna of pure Au, with no magnetic core. The horizontal axis of the graph indicates microns of distance downtrack from the center of the pole. The spot on the medium being heated is approximately 0.35 microns downtrack of the pole center. As can be seen, the magnetic field intensity distribution is essentially constant across the width of the pole, which lies between −0.3 and +0.2 microns (labeled MP). In the absence of the MCA (dashed line (25)), the magnetic field intensity decays sharply beyond the lateral dimensions of the pole.
In the presence of the MCA (solid line (20)), the magnetic field intensity rises (to approximately 10 kOe, compared to the value of approximately 4 kOe in the absence of the MCA) and peaks at approximately the trailing edge of the MCA, then has a sharp gradient at approximately 0.35 microns. However, the actual spot being heated is located at approximately 0.4 microns, which is at or beyond the outer edge of the plasmon generating layer (27) in FIG. 4. This indicates that the strongest field and steepest gradient of the magnetic field profile in the presence of the MCA is at the edge of the magnetic core ((24) in FIG. 4), while the actual spot being heated is at the edge of the generating layer ((27) in FIG. 4) that covers the core. These results indicate that to reduce the distance between the peak field and gradient and the peak point of heating, a thin MCA plasmon generating layer is preferred.
However, FIG. 6 shows the graphical simulation results of the transmitted power through the edge plasmon mode at various MCA plasmon layer thicknesses. In these simulations, the MCA is assumed to have a uniform core size and plasmon layer thickness along the MCA length. Both Ag alloy and Au films are considered. The power value indicated on the vertical axis of the graph is the percentage of the power transmitted in the MCA plasmon mode relative to that transmitted using a pure Au antenna. The figure indicates that as the plasmon layer thickness decreases, the efficiency of the coupling of the optical energy to the plasmon mode is significantly reduced. Thus, less heating of the medium is expected with thinner plasmon generating layers. Therefore, a trade-off exists between reducing the separation between the position of the magnetic field peak and the heating peak and achieving efficient heating of the recording medium. The prior arts cited above do not address this trade-off or methods of dealing with it advantageously.